1. Field of the Invention
This invention relates to a fundus examination device, and particularly to an improvement of an electronic ophthalmoscope which uses laser light as its light source, in which the laser beam is deflected for scanning in two dimensions to illuminate the fundus of the eye being examined, and light reflected from the eye fundus is photoelectrically detected and processed to obtain fundus information.
2. Description of the Prior Art
Conventionally, examination of the fundus of the eye is widely carried out by either the method of the physician directly examining the fundus of the patient's eye using a device called a funduscope or the method of photography using a special camera called a fundus camera. Furthermore, with advances in electronic technology in recent years, the photographic film of a conventional fundus camera is giving way to methods using a camera tube or other photoelectric transducer to directly obtain fundus information as an electrical signal, which may be then processed, stored or displayed on a TV monitor.
Against this background, the first laser-scanning electronic ophthalmoscope was developed in the United States (see U.S. Pat. No. 4,213,678 and Applied Optics, Vol. 19 (1980), p. 2,991) and had many benefits, attracting much attention.
Namely, a laser is used as a light source of a previously studied cathode-ray-tube flying-spot scanning fundus imaging device. Furthermore, light is permitted to go through only a small region in the center of the pupil, and light reflected from the eye fundus is obtained from a wide region around the pupil, photoelectrically converted and amplified to give low-illuminance yet high signal-to-noise ratio images of the eye fundus which can be projected onto TV monitors in real time. Moreover, the amount of fluorescent dye which must be intravenously injected during fluorescent fundus imaging may be greatly reduced. In addition, by modulating the incident scanning light, retinal function may be examined while observing images of the eye fundus, thus serving as a so-called fundus perimeter or fundus analyzer. Furthermore, that invention provides the potential of obtaining a superior diagnostic device from the standpoint of making the striking depth of field with the laser beam, eliminating corneal reflections due to polarization, employing its monochromaticity and providing expansion into therapeutic machines (coagulators).
This new type of ophthalmoscope has since been subject to many further experiments and improvements by research groups around the world. Among them, the present inventors have invented, developed and applied for a patent on an extremely innovative stereoscopic shape-measuring device based on a completely new principle which allows three-dimensional measurement of an eye fundus in vivo (see Japanese Patent Application Public Disclosure No. 1(1989)-113605, corresponding to U.S. Pat. No. 4,900,144). That invention employs a means of detecting displacement of the position of the focus of light reflected from the subject and a signal processing means which eliminates the effects of the optical reflection characteristics of the subject, thereby extracting three-dimensional stereoscopic information from the subject. The invention enables an extremely short measurement and processing time, high accuracy and reproducibility of measurement, and moreover, information regarding the normal two-dimensional reflection characteristics can be obtained at the same time as the three-dimensional information. Thus the invention provides superior technology for stereoscopic shape measurement of the in-vivo eye fundus in general and measurement of the optic disk for early diagnosis of glaucoma in particular.
However, when applying this stereoscopic shape-measuring device to an actual in-vivo eye fundus, as is apparent from considering the principle of measurement, the accuracy of measurement is degraded if the state of dilation of the pupil is poor. Namely, in the device disclosed in Japanese Patent Application Public Disclosure No. 1(1989)-113605), changes in the contour of the fundus subject to examination are detected as changes in the flux of light passing through detection slits, but if the state of dilation of the pupil is poor, then the changes in light flux are reduced. Therefore, determination of whether the three-dimensional shape data obtained with this device is valid or not necessitates a judgmental check of the state of dilation of the pupil. However, the device disclosed in Japanese Patent Application Public Disclosure No. 1(1989)-113605) does not provide a function for measuring the diameter of the pupil in the anterior portion of the eye, so the examiner must check the state of dilation of the patient's pupil either visually or using a separate method or device.
In addition, while unrelated to three-dimensional measurement, in normal two-dimensional fundus imaging methods typically using laser scanning, it is possible to display an image of the fundus on a monitor screen in real time using low-intensity illumination which causes little discomfort to the patient. However, when imaging the fundus using visible, short-wavelength laser light in particular, after positioning the device with respect to the eyeball as in conventional non-pupil-dilating fundus cameras, the eye is flash-illuminated with laser light for only the time required for one frame or several frames at a time to record an image of the fundus in memory. This imaging method has been found to be effective from the standpoint of reducing discomfort in the patient. In this case, if the positioning of the device to the eyeball is insufficient, flare due to light reflected from the anterior portion of the eye will impinge on the image of the fundus and result in a loss of picture quality. Although the process of observing the anterior portion of the eye is important for positioning the device to the eyeball, conventional laser-scanning fundus examination devices are not provided with functions for observing the anterior portion of the eye, making the adjustment of the device position with respect to the eyeball difficult.
Naturally, techniques for observing the anterior portion of the eye are employed in the traditional non-pupil-dilating fundus cameras of the past, namely by redirecting the light path somewhere in the midst of the camera's optical system or by inserting an auxiliary lens into the optical system and thereby observing the anterior portion of the eye. Naturally, these sorts of techniques can also be employed in the laser-scanning type of fundus imaging system. However, these traditional techniques involve the bother of mechanically redirecting the light path or inserting and removing lenses and moreover the fundus and the anterior portion of the eye cannot be observed at the same time, so they are not necessarily the ideal solution.